1. Field of the Invention
The present invention relates to a permanent magnet type synchronous motor.
2. The Related Art
A permanent magnet type synchronous motor includes a rotatably supported rotor and a stator arranged around the rotor. The rotor includes an iron core and permanent magnets arranged circumferentially around the circumference direction of the iron core. The stator also includes an iron core, with coils arranged in a plurality of locations on the circumference of the stator iron core.
In a permanent magnet type synchronous motor of a surface magnet type, having sector-shaped permanent magnets exposed at the outer circumferential edge of the rotor iron core, the relative magnetic permeability of the permanent magnets is substantially 1 so that the effective air gap as taken from the stator side, is the sum of the gap between the stator and the rotor and the thickness of the permanent magnets. This lowers the reaction of the stator to make field-weakening control (by control of current to the coils in the stator) difficult.
There has been disclosed (in Published Japanese Patent Application laid-open No. 8-107639) a permanent magnet type synchronous motor of an internal magnet type in which the permanent magnets are buried within the rotor iron core. In this case, the effective air gap, as taken from the stator side, is equal to the gap between the stator and the rotor so that the responsiveness of the stator is increased to facilitate field-weakening control.
Since permanent magnets having a high magnetic resistance are arranged on a d-axis magnetic path, and not on a q-axis magnetic path, the magnetic resistance changes with the circumferential position so that a q-axis inductance Lq grows larger than a d-axis inductance Ld to thereby exhibit counter saliency. By feeding a proper d-axis current, therefore, it is possible to generate not only the magnet torque on the basis of the magnetic flux of the permanent magnets and the q-axis current, but also the reaction torque, to thereby reduce the q-axis current necessary for generating the torque.
In the aforementioned permanent magnet type synchronous motor, however, the portions of the iron core radially outward of the permanent magnets must be enlarged so that a sufficient strength may be provided against the centrifugal force exerted on the permanent magnets when the synchronous motor is rotated at a high speed. However, both the centrifugal force due to the weight of the rotor portions radially outward of the permanent magnets and the centrifugal force generated in the permanent magnets are exerted on the relative thin portions of the core radially outward of the end portions of the permanent magnets, so that the strength of the rotor iron core is lowered.
It is, therefore, conceivable to increase the thickness of the rotor portions radially outward of the end portions of the permanent magnets. In this case, however, the leakage of magnetic flux increases so much that a high torque cannot be generated and, moreover, a distortion is caused in the magnetic flux distribution in the gaps, which distortion generates harmonic components in the magnetic flux density waveforms and thereby causes torque oscillations.